ROTATION OF CROPS. 409 



order that this object may be effectually obtained, there are certain 

 principles which ought to be adopted as guides. The chief of these is 

 to be derived from a knowledge of what specific benefit or injury every 

 culinary plant does to the soil, with reference to any other culinary 

 plant. It ought to be known whether particular plants injure the soil 

 by exhausting it of particular principles, or whether the soil is ren- 

 dered unfit for the growth of the same, or any allied species, by excre- 

 tions from the roots of plants, while the same excretions, acting in the 

 way of manure, add to the fitness of the soil for the production of other 

 species. The excretory theory is now generally abandoned, and it is 

 believed that plants exhaust the soil generally of vegetable food, par- 

 ticularly of that kind of food which is peculiar to the species growing 

 on it for the time being. For example, both potatoes and onions 

 exhaust the soil generally, while the potato deprives it of something that 

 is necessary to ensure the reproduction of good crops of potatoes, and 

 the onion of something which is necessary for the reproduction of large 

 crops of onions. According to the theory of De Candolle, both crops 

 exhaust the soil generally, and both render it unfit for the particular 

 kind of crop ; but this injury, according to his hypothesis, is not 

 effected by depriving the soil of the particular kind of nutri- 

 ment necessary for the particular kind of species, but by excreting 

 into it substances peculiar to the species with which it has been 

 cropped, which substances render it unfit for having these crops 

 repeated. Both these theories are attended with some difficulty 

 in the case of plants which remain a great many years on the 

 same soil, as, for example, perennial-rooted herbaceous plants and 

 trees. The difficulty, however, is got over in both systems. By the 

 first, or old theory, the annual dropping and decay of the foliage are 

 said to supply at once general nourishment and particular nourish- 

 ment; and by the second, or new theory, the same dropping of the 

 leaves, by the general nourishment which it supplies, is said to neutralize 

 the particular excretions. A wood of the pine or fir tribe standing so 

 thick that their roots will form a network under the surface, will 

 not poison each other ; but remove these trees, and place a new plan- 

 tation on the same soil and they will not thrive, owing, as we think, to 

 the principles most conducive to the growth of coniferous trees being 

 exhausted, as is explained by Liebig. The practical inference from 

 either theory is much the same that is, a change of crops, the rules 

 for which adopted by the best gardeners are as follows : 



1 . Crops of plants belonging to the same natural order or tribe, or 

 to the natural order and tribe most nearly allied to them, should not 

 follow each other. Thus, turnips should not follow any of the cabbage 

 tribe, sea-kale, or horse-radish, nor should peas come after beans. 



2. Plants which draw their nourishment chiefly from the surface of 

 the soil should not follow each other, but should alternate with those 

 which draw their nourishment in great part from the subsoil. Hence, 

 carrots and beets should not follow each other: nor onions and 

 potatoes. 



